The present invention relates to the field of micro-electromechanical devices. More particularly, the present invention provides micro-electromechanical devices, substrate assemblies for forming such devices, and methods of manufacturing the substrate assemblies and devices.
Micro-electromechanical devices such as pressure sensors, actuators, etc. provide advantages in many different applications. Two basic approaches have been developed to manufacture the devices using many well-known conventional integrated circuit manufacturing techniques. The two basic approaches are typically referred to as surface mnicromachining and backside bulk micromachining.
Conventional surface-micromachining technologies do, however, have several tradeoffs. Surface micromachining typically has the advantage of superior registration since all layers are defined on the same side of the wafer using conventional planar lithographic processes. The transducer elements (piezoelectric capacitors, piezoresistive elements, etc.) can be aligned precisely to the same features that define the mechanical structure being formed. Moreover, because definition of the mechanical structure is performed with standard integrated circuit (IC) processes, the features can be transferred in a repeatable and precise manner. Good alignment of transducer elements to a high-resolution mechanical structure creates devices with high efficiency (performance) and repeatable characteristics.
Surface micromachining processes do, however, typically require encapsulation layers that are impervious to very aggressive chemicals used in forming the features on the front surface of the devices. By definition, surface-micromachining exposes the surface of the wafer to a variety of etchants and other removal processes. For example, in the case of sacrificial phosphosilicate glass (PSG) layers, the encapsulation layer may need to protect all vulnerable layers from highly-concentrated hydrofluoric acid (HF) liquid or vapor. In the case of sacrificial polysilicon and/or silicon layers, the encapsulation layer may need to protect all other materials from a heated potassium-hydroxide (KOH) or tetramethyl-ammonium-hydroxide (TMAH) solution.
Regardless, the encapsulation layer typically adds to the bulk of the devices and may also affect performance by, for example, increasing the stiffness of a diaphragm used in a pressure sensor or actuator. That additional stiffness may reduce sensitivity (in the case of, e.g., a sensor) and/or it may increase the power requirements for operating an actuator. To address these issues, it may be desirable to reduce the thickness of an encapsulation layer. Reducing the encapsulation layer thickness, however, increases the likelihood that the underlying features on the device will not be adequately protected, thereby reducing product yield. As a result, surface micromachined device performance is often limited by availability of a thin film technology suitable for encapsulation layers.
Conventional backside bulk-micromachining techniques also have tradeoffs. The essence of backside silicon micromachining separates the machining operation from the fine-featured frontside. However, this same characteristic also leads to the limitations of backside bulk-micromachining. When the machining proceeds from the backside, front-to-backside alignment defines the registration between the mechanical structure and the transducer elements. Generally, the overlay capabilities are coarse and performance is lost.
Perhaps even more significant, the silicon etch proceeding from the backside will reach the front side at a position dependent on the wafer thickness and/or etch profile distribution. For potassium hydroxide (KOH) etching with anisotropic sidewall angle consistently near 53xc2x0, each 50 xcexcm variation in wafer thickness will result in about 30 xcexcm variation in the finished position of the mechanical feature at the front. In the case of Deep Reactive Ion Etching (DRIE), the etch profile varies with feature size and wafer position so that variations in finished dimensions can also be on the order of tens or even hundreds of microns within a single wafer. Combining alignment errors and etch profile variations, bulk machining techniques often lead to large discrepancies between the frontside-defined transducer elements and the mechanical structure.
The present invention provides micro-electromechanical devices, substrate assemblies from which the devices can be manufactured, and methods to manufacture the devices. The invention combines the advantages of conventional surface and bulk micromachining processes to create an integrated micro-electromechanical system (MEMS) technology that provides high performance, high yield, and manufacturing tolerance. The devices manufactured according to the present invention include, but are not limited to, pressure sensors, vibration sensors, accelerometers, gas or liquid pumps, flow sensors, resonant devices, and infrared detectors.
One advantage of the present invention is that the mechanical integrity of the substrates on which the devices are formed is maintained until the last processing steps. By maintaining the mechanical integrity of the substrate while all of the front side processing is performed accuracy in the alignment of the various structures on the device can be improved as compared to known methods of manufacturing such devices. As discussed above, in methods in which voids are formed in the substrate before all of the front side processing is complete (including metallic contacts), alignment and yield can suffer due to the reduced mechanical integrity of the underlying substrate.
Another potential advantage of the present invention is that the fatigue resistance of the devices may be improved by maintaining an included angle of less than 90 degrees between the diaphragm layer and the substrate in the devices.
Because this invention relates to methods of fabrication that could be applied to a wide variety of micro-fabricated sensor and/or actuator devices, the scope of application is very wide. Nonlimiting examples of applications in which vibration sensors or accelerometers of the present invention may be used include navigational systems (automotive, aeronautic, personal, etc.); environmental monitors (seismic activity, traffic monitors, etc.); equipment monitors (industrial equipment, etc.); component monitors (fatigue/crack detection, shock threshold detection, etc.); and biomedical monitors (cardiac monitor, activity monitor, ultrasonic-GPS, etc.).
Nonlimiting examples of applications in which pressure sensors of the present invention may be used include aeronautics (altimeter, air velocity, etc.); combustion engine applications (combustion diagnostic, exhaust monitor, fuel monitor; etc.); and auditory applications (hearing aids, mini-microphones, etc.).
Nonlimiting examples of applications in which resonant structures of the present invention may be used include chemical sensing (electronic nose, military, biomedical, etc.) and environmental monitors (humidity, biohazard detection, pressure, etc.).
Nonlimiting examples of applications in which pumps of the present invention may be used include miniature vacuum systems (mass spectrometry, medical diagnostics, etc.); drug delivery (implanted drug delivery, precision external delivery, etc.); microfluidics (DNA chips, medical diagnostics, etc.); and sample extraction (environmental, biomedical, etc.).
In one aspect, the present invention provides a method of manufacturing a micro-electromechanical device having front and back sides, the method including providing a substrate having a first side located proximate the front side of the device and second side proximate the back side of the device; providing sacrificial material on a selected area of the first side of the substrate; providing a diaphragm layer on the sacrificial material and the first side of the substrate surrounding the sacrificial material in the selected area; providing at least one transducer on the front side of the device, the transducer located over the sacrificial material, wherein the transducer includes transducing material and electrical contacts in electrical communication with the transducing material; forming a void in the substrate from the second side of the substrate towards the first side of the substrate after providing the transducer on the front side of the device, wherein at least a portion of the sacrificial material is exposed within the void proximate the first side of the substrate; and removing at least a portion of the sacrificial material through the void, wherein a portion of the diaphragm layer is suspended directly above the substrate within the selected area.
In another aspect, the present invention provides a substrate assembly having front and back sides, the assembly including a substrate having a first side located proximate the front side of the device and second side proximate the back side of the substrate assembly; sacrificial material on the first side of the substrate in a plurality of selected areas; a diaphragm layer covering the sacrificial material in the selected areas, the diaphragm layer extending to cover the first side of the substrate surrounding the sacrificial material in the selected areas; a plurality of transducers on the front side of the device, each of the transducers located over at least a portion of each of the selected areas, wherein the transducer includes transducing material and electrical contacts in electrical communication with the transducing material; wherein the sacrificial material in the selected areas is encapsulated between the substrate and the diaphragm layer. This substrate assembly can then be separated into a plurality of MEMS devices, each device including at least one of the transducers.
In another aspect, the present invention provides a micro-electromechanical device having front and back sides, the device including a substrate having a first side located proximate the front side of the device and second side proximate the back side of the device; a void formed through the first and second sides of the substrate, the void including an opening proximate the first side of the substrate; and a diaphragm layer spanning the opening in the first side of the substrate and attached to the first side of the substrate, wherein a portion of the diaphragm layer is suspended directly above a portion of the substrate surrounding the opening of the void; and wherein the suspended portion of the diaphragm layer and the substrate form an included angle at their junction of less than 90 degrees.
These and other features and advantages of the invention are described more completely with respect to various illustrative embodiments below.